i fixed a similar problem with a better ignition
spent years messin with carbs....whilst they may have played a part once i got my ignition sorted fewer problems and progress made quickly.
flat spot/stumble....= can't ignite it.... once it does, regardless of how rich or lean it might be no flat spot....
then you can observe what other additional minor problem there is and set up the cab properly
by way of experiment only-----(for the time being) put the vacuum advance to manifold vacuum
drive gently from idle through the problem area
at 40 mph its probably only doing 1500 rpm and it won't need much throttle, leaving the vacuum advance slightly in the mix...
it may ping as you load it up, but if it fixes the "dead spot" sure fire indicator that it is ignition/curve related
he then has a choice
set it up with vacuum advance on ported vacuum (70s 80s OEM emissions era style)
set it up with vacuum advance on manifold vacuum (50s OEM who cares about emissions? style)
get his curve changed for pure mechanical advance (what everyone does if they can't get it to work or don't want, vacuum advance)
get an ignition with a higher spark power and open up the plug gaps slightly
i'm presuming street rather than drag racing
his 15 initial would indicate that some care and attention has been paid to initial advance and advance curve in the past
i've made a few assumptions about the set up that may be wrong
if it works when accelerating hard id guess the pump shot is alright i.e not too much, at lower vacuum and higher air flow....
makes me wonder if it leans out when accelerating gently or cruising.
Dave
